1. Technical Field of the Invention
The present invention relates to mobile cellular telephone communications systems and, in particular, to overcoming signal path interference during the delivery of messages to mobile stations by increasing the power of the communications transmissions.
2. Description of Related Art
Historically, paging systems and mobile telephone systems have provided service to a limited geographic area using a relatively high-powered centralized transmitting site. This arrangement has worked well in small cities and municipalities, but has often proved unsuitable for large metropolitan areas. To adequately serve a large geographic area, cellular phone systems began to simulcast (transmit the same signals at substantially the same time) from several transmitters strategically positioned to provide wide-area coverage. In such systems, a pager or a mobile station could receive information anywhere in the specified wide area.
Today, the trend in wireless communications is to provide even greater geographic coverage. Nationwide communication systems are often contemplated in an attempt to provide communication services to customers without regard to where they are in the United States, or for that matter, in the world. One known communication system employs satellites to transmit paging messages to pagers as well as cellular mobile stations.
It is well known that, in ground based cellular communication systems or satellite based communication systems, signal path interference can block a signal and prevent it from reaching the specified receiver. Such signal path interference of this type is often due to structural blockage from thick foliage, buildings and the like. Generally speaking, the signal path interference results in a mobile station being in an operational mode but also being temporarily unable to communicate with a transceiver.
Many methods and systems have been proposed for ground based as well as satellite based communication systems in response to the problem of mobile station unavailability for message delivery due to signal path interference. One proposed system includes a store in a mobile switching center for storing a message until the presence of the mobile station is detected through a registration, at which point the message is delivered or made available for retrieval. A drawback to this approach, however, is that the delivery of important messages may be unacceptably delayed until the mobile station clears the signal interference and re-registers within a cell area. Another drawback is that no further attempts to communicate with the mobile station and to deliver the potentially important message are made by the mobile switching center transceiver until re-registration occurs.
Another proposed solution to the problem of mobile station unavailability owing to signal path interference is to increase the transmission power level within a "standard" range of power levels. Increasing the power level within a standard range, however, may not be sufficient to overcome the signal interference due to structural path blockage. Moreover, current proposals for increasing transmission power include increasing power either on a system wide basis or on a cell wide basis. Under these proposals, unnecessary power levels are utilized for transmission to mobile stations which are receiving signals clearly in order to reach those mobile stations that are not receiving signals clearly.
In proposed satellite based communications systems, a mobile station located toward the center of a building or under a large oak tree with thick foliage may be subject to signal path interference and may not be able to receive and respond to a message delivery attempt or to a paging attempt from a satellite based transceiver. With satellites, the solution of transmitting at increased power levels is not always possible because many satellites have fixed transmission power level capabilities. While some satellites can exceed the standard transmission power levels to attempt to deliver a message, or to page a mobile station, other satellites have no capability of transmitting at increased power levels. Thus, a given satellite serving the mobile station may not be capable of performing a high power level transmission to overcome signal path interference.
Even for those scenarios in which it is possible to exceed standard transmission power levels, economic disincentives exist for doing so. It is likely that only some users will be interested in selectively using a higher power transmission level service. Other users simply may not care about becoming more "reachable". It would be unfair for the expenses of increased power transmission levels to be borne by all mobile station users. Thus, even if a satellite based transceiver has the capability to transmit a message at an increased power level, the increased power level should not be provided to those users that would not ordinarily need or appreciate such a service feature. There is a need, therefore, for a system to only transmit at increased transmission power levels in select situations.
Timing problems also exist with using satellite based transceivers for increased power transmissions to a ground based receiver. First, in order to avoid unnecessary transmissions at high power levels, increased power level transmissions should only be made after a transmission at a standard power level fails. Second, in a typical communication system, namely one in which high penetration power level transmissions do not occur, a paging system may designate that a user is unavailable after a mobile station fails to respond to at least one page attempt. Under current systems, certain time out values are established which only account for the amount of time required to page a mobile station at a standard power level. Current systems are set up wherein an originating source of a message rebroadcasts its message to initiate paging once a time out value is reached.
Unfortunately, such timeout values are frequently less than the time required for the communication system to initiate and process a second paging attempt or a third paging attempt at higher power levels. Accordingly, there is a need to reduce or eliminate unnecessary subsequent broadcast attempts or paging attempts by the originating source until it can be determined that a receiver is not responding even to the high power paging or communication attempts.
Another problem that exists, in those systems which utilize high power transmissions for paging or message delivery, is that a user has no way of knowing that a system is having to use non-standard transmission power levels to reach the user due to signal path interference. Accordingly, there is a need to provide such information to a user so that a user will know to move to a location less subject to signal path interference.